In the semiconductor wafer and substrate processing fields various types of processing equipment are now commonplace which provide for automated handling of wafers in a vacuum environment. A paramount concern in the handling and processing of such wafer substrates is the need to minimize particle generation that could contaminate the wafers damaging the devices being formed thereon. For this reason, most semiconductor device fabrication is conducted within a "clean room" environment where extreme measures are taken to minimize the presence of particulate matter.
It is known that the mechanical movement of machine parts is responsible for generation of unwanted particles and, accordingly, the movement of various automated wafer transport and stationing mechanisms within wafer processing equipment is carefully controlled. Many types of wafer processing equipment are designed to process wafers individually, i.e., one at a time or serially, as opposed to batch processing. In such equipment, wafers are typically loaded in the processing system by a cassette and individually and automatically transferred from the cassette to one or more processing stations. A number of systems have recently been developed with a plurality of processing stations housed within a single vacuum environment. The stations may perform processing steps such as, for example, sputtering, chemical vapor deposition (CVD), plasma etching, heating, etc. These systems typically include one or more load-lock chambers where the wafers are introduced and removed from the vacuum environment. In so-called "cluster tool" systems, the wafers are typically transferred by means, such as robot arms, between the load-lock chamber and the process stations within the vacuum environment.
At a process station, it may be necessary to hold the wafer firmly against a support surface with a clamp during processing, such as for example to maintain the position of the wafer relative to the processing equipment or to maintain good thermal contact to a heat transfer element. A common technique for maintaining the temperature of a wafer undergoing processing in a vacuum environment is to introduce a conductive gas in a narrow space at the backside of the wafer, thereby thermally coupling the wafer to a temperature control element. When using a backside gas, which is introduced at a pressure higher than the ambient pressure of the processing chamber, clamp means are required to ensure that the backside gas does not move the wafer off of the support surface.
In most cluster tool systems, and many other types of semiconductor processing equipment, the wafers are loaded onto and removed from the process platform in a horizontal orientation. This allows gravity to be used to hold the wafer in place on the surface of the platform while the wafer is clamped and unclamped. Thus, unclamping normally involves simply translating the clamp mechanism in an upward direction, relying on gravity to cause separation of the wafer and the clamp.
Unfortunately, the particular process used in a process chamber may cause the clamp to adhere to a wafer after completion of processing of the wafer at the process station, thus preventing the wafer from being picked up by the robot arm or other transport mechanism. This may occur, for example, when depositing a metal layer over a wafer which causes the wafer to stick to the clamp. Another example is when a wafer having a top layer of a material which melts or becomes plastic at an elevated temperature used in a processing step, causing the top layer to adhere to the clamp.
When a wafer sticks to a clamp, the processing system typically must be shut down to free the stuck wafer, a procedure that normally involves manual intervention at atmospheric pressure. After the process chamber is vented to atmosphere to permit manual wafer removal, it can take several hours before the chamber can be placed back in service due to the need to pump down the chamber and allow outgassing of contaminants (e.g., water vapor, etc.) that have become adsorbed to the chamber surfaces. Thus, it can easily take four to five hours to recover from a stuck wafer.
Of particular concern to the present invention are sputtering systems where a layer of metal is formed over a wafer or substrate at a controlled elevated temperature. As described above, when sputter coating a silicon wafer, it is advantageous to clamp the wafer to the heater table or platform to allow the introduction of a backside gas. This gas serves as a heat transfer medium in the vacuum system to maintain the temperature of the wafer within a desired temperature range during the sputtering operation. On occasion, the wafer sticks to the clamp when the sputtering system tries to un-clamp the wafer, thereby preventing the transfer of the wafer to the robot arm which will carry the wafer to the load-lock station or to the next process station. The above-described manual intervention operation must be used, usually causing several hours of lost processing time. The lost time decreases the wafer throughput of the system, typically causing revenues losses for the user due to the decreased throughput.
Prior art processing systems have either not addressed this problem or have addressed it in a way which is not entirely satisfactory. One approach to minimize the possibility of sticking uses clamps having contact surfaces which are slightly beveled to cause the clamp to only contact at the outside edge of the wafer, as far away from the deposition process as possible. An overhang formed by the projection of the beveled edge may be used to block sputtered material from being deposited in the area where the clamp makes contact with the wafer. However, the beveled surface does not completely eliminate the possibility of sticking because, with time, sputtered material makes its way under the clamp's edge and back to the area of the clamp surface which contacts the wafer's edge. It has been found that such a system may experience one wafer sticking per one thousand wafers processed. This level is in need of improvement. Moreover, as indicated above, there are some instances where material has been previously deposited at the edge of the wafer. When such wafers are clamped for further processing, the possibility of sticking can be considerably higher. Accordingly, there is a need to substantially reduce the number of incidents in which a wafer or substrate becomes stuck to a clamp in such systems.